What is the role of a transformer-rectifier unit (TRU) on the 115V AC/400 Hz side of the system?

• A. It steps down and rectifies AC to produce the aircraft's 28V DC for the DC buses and charging
• B. It amplifies 115V AC to support cabin power distribution
• C. It converts DC to 115V AC for main distribution
• D. It isolates fault currents between buses without changing voltage

Answer: (A)

The transformer-rectifier unit on the 115V/400 Hz side is there to convert that high-voltage AC into the aircraft’s DC power. It does this in two stages: first the transformer steps the 115V AC down to a lower AC level, then a rectifier converts that lower AC into DC. The result is a regulated 28V DC that feeds the DC buses and provides battery charging when AC power is available. This setup supplies the essential 28V DC power for avionics, controls, and lighting, and also keeps the batteries charged. The other choices describe functions handled by other equipment (inverters for DC-to-AC, DC isolation or different converters), not what the TRU does.